Optimal Dispersion Under Asynchrony

TL;DR

This paper presents an optimal $O(k)$-time dispersion algorithm for mobile agents in anonymous graphs, using minimal memory, and introduces a novel technique for constructing port-one trees.

Contribution

It provides the first optimal $O(k)$-time asynchronous dispersion algorithm with efficient memory use, closing a key complexity gap in the field.

Findings

Achieves optimal $O(k)$ dispersion time in asynchronous setting.

Uses $O( ext{log}(k+ ext{Δ}))$ bits of memory per agent.

Introduces a new technique for constructing port-one trees in anonymous graphs.

Abstract

We study the dispersion problem in anonymous port-labeled graphs: $k \leq n$ mobile agents, each with a unique ID and initially located arbitrarily on the nodes of an $n$-node graph with maximum degree $\Delta$, must autonomously relocate so that no node hosts more than one agent. Dispersion serves as a fundamental task in distributed computing of mobile agents, and its complexity stems from key challenges in local coordination under anonymity and limited memory. The goal is to minimize both the time to achieve dispersion and the memory required per agent. It is known that any algorithm requires $\Omega(k)$ time in the worst case, and $\Omega(\log k)$ bits of memory per agent. A recent result [SPAA'25] gives an optimal $O(k)$-time algorithm in the synchronous setting and an $O(k \log k)$-time algorithm in the asynchronous setting, both using $O(\log(k+\Delta))$ bits. In this paper, we close the complexity gap in the asynchronous setting by presenting the first dispersion algorithm that runs in optimal $O(k)$ time using $O(\log(k+\Delta))$ bits of memory per agent. Our solution is based on a novel technique we develop in this paper that constructs a port-one tree in anonymous graphs, which may be of independent interest.